1. Field of the Invention
The present invention relates to a pattern forming method based on immersion lithography, and a method of manufacturing a semiconductor device.
2. Description of the Related Art
Development of exposure tools has been in progress along with shrinkage of semiconductor devices. A 157 nm-lithography tool has been developed as a next-generation lithography tool, which replaces a tool presently in practical use to 193 nm lithography. However, due to delays in development of exposure tools and resist materials, a 193 nm-immersion lithography exposure tool is regarded as reliable at present. In the immersion lithography exposure tool, a solvent such as water which is a medium having a refractive index greater than that of air is filled between an objective lens and resist films targeted for image formation to increase a critical angle of an interface between the objective lens and the medium or an interface between the medium and the resist films, and an appropriate lens is used to enable image formation of a small pattern with a larger diffraction angle (refer to pamphlet for International publication No. WO99/49504).
In an optical lithography step where the immersion lithography exposure tool is used to transfer a pattern to a resist film, there is concern over the elution of an inclusion from the resist film into a liquid immersion fluid. This arises from the fear that a photo-acid generator contained in the resist film and its photoproducts which are photo-generated acids, basic materials and other low-molecular components elute into the immersion fluid and contaminate the element of the projection optics which contacts with the liquid immersion fluid, a shower head, a wafer stage, and so forth. The adsorption of the eluting substances onto the projection optics or corrosion thereof emerges as an optical path difference, and therefore influences imaging performance as aberration and flare, and it is presumed that this influence accumulatively increases due to an increase in processing for the resist processes of the same kind. On the other hand, the elusion of the substance into the immersion fluid changes the refractive index of the immersion fluid and thus changes the optical path length, which causes aberration of a projected image and affects the imaging performance.
Furthermore, in an immersion lithography exposure tool which selectively fills the liquid immersion fluid between a downstream surface of an optical axis of the objective lens and a substrate upper surface targeted for image projection, the following problems occur in a case where the liquid immersion fluid remains on the substrate when the substrate is loaded after completion of an exposure step.
1. If the liquid immersion fluid leaks into the exposure tool and a coating tool and touches electric systems therein, trouble occurs in the operation of the tools. This results in a reduction in productivity and generation of costs due to the stoppage and repair of the tools, and a loss due to the reprocessing and damage of a wafer.
2. If the liquid immersion fluid leaks into the exposure tool and the coating tool and is left as it is, microorganisms such as bacteria are grown in the leaking solution, which increases the concentration of a chemical species such as a base in the air inside the exposure tool and coating tool. This leads to a decrease in productivity due to deterioration of lithographic performance, stops the tools and generates repair cost.
3. If drying is performed before post exposure bake (PEB) with the liquid immersion fluid remaining on the substrate surface or if the PEB step is implemented with the remaining liquid immersion fluid, watermarks are produced on the resist film. In a relevant part, there are caused non-resolution of the resist pattern, a T-topped shape and a size change. Defects are produced in a process pattern using this resist pattern as a mask. This results in a productivity decrease.
4. If the PEB is carried out with the liquid immersion fluid remaining on the substrate surface, the temperature of the resist film changes due to latent heat in the vicinity of the part where the liquid immersion fluid remains, and the size of the resist pattern changes. This results in a productivity decrease.
5. If the PEB is carried out with the liquid immersion fluid remaining on the substrate surface, the amount of a substance volatizing from the film on the substrate increases, or a substance is generated by a reaction between the volatilizing substance and the liquid immersion fluid. As a result, adherents on the inner wall of a baking unit in the PEB step increase, and the adherents act on a wafer as dust to cause a pattern defect, leading to a productivity decrease and a loss.
6. If the PEB is carried out with the liquid immersion fluid remaining on the substrate surface, a surface state change is caused in the resist film or a cover film in the vicinity of the part where the liquid immersion fluid remains. This causes the removal of the cover film, an insufficient removal of the cover film due to occurrence of parts of different development characteristics, or a loss from a productivity decrease due to occurrence of development defects.
Therefore, there has been a demand to realize a pattern forming method and a method of manufacturing a semiconductor device which can restrain the occurrence of the problems caused by the liquid immersion fluid used during the immersion lithography exposure.